1. Field of the Invention
The present invention relates to a matrix driving method of a liquid crystal display apparatus using a ferroelectric liquid crystal and, more particularly, to a liquid crystal display apparatus for executing a gradation display and to a method of driving such an apparatus.
2. Related Background Art
With respect to a display device using a ferroelectric liquid crystal (FLC), as shown in JP-A-61-94023 or the like, there is known a display device in which a ferroelectric liquid crystal is injected into a liquid crystal cell constructed by confronting glass substrates in which transparent electrodes are formed on two inner surfaces so as to keep a cell gap of about one to three microns and an orientation process has been performed.
It is a feature of the display device using the ferroelectric liquid crystal that since the ferroelectric liquid crystal has a spontaneous polarization, a coupling force between an external electric field and the spontaneous polarization can be used for switching and that since the direction of the major axis of the ferroelectric liquid crystal molecule corresponds to the polarizing direction of the spontaneous polarization in a one-to-one corresponding manner, a switching operation can be performed by the polarity of the external electric field.
Since the ferroelectric liquid crystal generally uses a chiral smectic liquid crystal (SmC*, SmH*), the major axis of the liquid crystal molecule in a bulk state exhibits a twisted orientation. However, the twisted state of the major axis of the liquid crystal molecule can be eliminated by inserting the liquid crystal into the cell having a cell gap of about 1 to 3 microns as mentioned above. (N. A. Clark et al., "MCLC", Vol. 94, pages 213 to 234, 1983)
A structure of an actual ferroelectric liquid crystal cell uses a simple matrix substrate as shown in FIG. 2.
The ferroelectric liquid crystal is mainly used as a binary (white/black) display device by setting two stable states into a light transmission state and a light shielding state. However, a multivalue, that is, a half-tone display can be also performed. As one of the half-tone display methods, an intermediate light transmission state is produced by controlling an area ratio of a bistable state in a pixel. The above method (area modulating method) will now be described in detail hereinbelow.
FIG. 4 is a diagram schematically showing the relation between the switching pulse amplitude of the ferroelectric liquid crystal device and the transmittance. FIG. 4 is a graph in which a transmission light quantity I after a single pulse of uni-polarity was applied to a cell (element) which has originally been set in the complete light shielding (black) state has been plotted as a function of an amplitude V of the single pulse. When the pulse amplitude V is equal to or less than a threshold value V.sub.th (V.ltoreq.V.sub.th), the transmission light quantity doesn't change. As shown in FIG. 5(b), the transmission state after the pulse was applied is equal to that of FIG. 5(a) showing the state before the pulse is applied. When the pulse amplitude exceeds the threshold value (V.sub.th &lt;V&lt;V.sub.sat), a state of a part in the pixel is shifted to the other stable state, namely, the light transmission state shown in FIG. 5(c) and exhibits an intermediate transmission light quantity as a whole. Further, when the pulse amplitude V increases and is equal to or higher than a saturation value V.sub.sat (V.sub.sat .ltoreq.V), as shown in FIG. 5(d), the light transmission quantity reaches a constant value because the whole pixel is set into the light transmission state.
That is, the area modulating method intends to display a half-tone by controlling the voltage so that the pulse amplitude V lies within a range of V.sub.th &lt;V&lt;V.sub.sat.
However, the area modulating method has the following serious drawback as will be explained hereinafter. Since the relation between the voltage and the transmission light quantity shown in FIG. 4 depends on a thickness of cell and a temperature, in other words, if there is a cell thickness distribution or a temperature distribution in the display panel, a different gradation level is displayed for the applied pulse of the same voltage amplitude. FIG. 6 is a diagram for explaining such a drawback and is a graph showing the relation between the voltage amplitude V and the transmission light quantity I in a manner similar to FIG. 4. FIG. 6 shows two curves showing the relations at different temperatures: that is, a curve H indicative of the relation at a high temperature and a curve L indicative of the relation at a low temperature. In the display (display device) of a large display size, a temperature distribution often occurs in the same panel (display section). Therefore, even if the operator tries to display a half-tone at a certain voltage V.sub.ap, a variation of the half-tone level occurs in a range from I.sub.1 to I.sub.2 as shown in FIG. 6, so that a uniform display state cannot be derived. Generally, since a switching voltage of the ferroelectric liquid crystal is high at a low temperature and is low at a high temperature and a difference between the switching voltages depends on a temperature change of a viscosity of liquid crystal, such a difference is ordinarily extremely larger than that of the conventional TN type liquid crystal device. Therefore, a fluctuation of the gradation level by the temperature distribution is fairly larger than that of the TN liquid crystal. Such a point becomes the maximum cause which makes it difficult to realize the gradation display of the ferroelectric liquid crystal device.
The above influences increase as an area of the liquid crystal cell increases (a variation of the cell thickness and a variation of the temperature easily occur), so that the gradation, especially, the analog gradation display in the cell of a large area using the FLC is impossible.